The salads in total are $9.50
The waters in total are $3.30
together the total cost is $12.80
First, convert the 15% to an actual number that can be used in a calculation. For percents, this is always done by simply dividing the percent (in this case 15%) by 100%. So, the conversational term "15%" becomes 15% / 100% = 0.15 in terms of a real mathematical number.
Second, you need to find out what 15% of your $12.80 meal cost is. This is always done by multiplying 0.15 by $12.80, or
0.15 x $12.80=$1.92.
So, the amount of tip you are going to leave is $1.92.
This makes the total cost of your meal (to write on your charge slip or other payment)
$12.80 + $1.92 = $14.72.
